This invention relates to ultrasonic imaging and more particularly to suppression of spurious artifact signals at ranges close to an excitation source, herein known as ring-down artifact.
Ring-down artifact is caused by transients associated with an exciter which cause interference with informational signals reflected from sources close to the exciter (echo signals). In close-in imaging, such as in intravascular structures, undesired ring-down artifact can impede accurate imaging.
One known mechanism for eliminating ring-down artifact is to gate on the echo signal so that all artifacts are eliminated in the close-in region where ring-down is expected to occur. However, useful echo signals are also eliminated by gating.
Another method described in U.S. Pat. No. 5,601,082, the disclosure of which is herein incorporated by reference, is to generate a reference scan to develop a long-term average and use the reference scan to subtract on all but useful echo signals. However, subtraction of a reference scan may also remove useful echoes having a time constant of the same order of magnitude as the averaged reference scan. Thus subtraction based on a simple reference scan is inadequate to analyze a full range of signal types. What is needed is a more accurate technique for identifying ring-down artifact so it can be separated from legitimate signals.
According to the invention, in an ultrasonic in-vivo imaging system, ring-down artifact is reduced or eliminated by dynamically enhancing the ring-down over a plurality of scans, and then determining the ring-down range by keying on a ring-down-to-blood transition characterized by a rapid change from high amplitude to low amplitude echoes. A ring-down pattern is computed for a single or several A-scans within the ring-down range, using for example an FFT analysis, and then selectively filtering subsequent images using the recently computed ring-down pattern.
In one exemplary embodiment, the invention provides a method for filtering an in-vivo ultrasonic signal. According to the method, an ultrasonic signal is emitted and a return signal is collected which includes at least an artifact component and a blood component. A transition region in the collected return signal is then identified, with the transition region having the artifact component and the artifact component combined with the blood component. A ring-down pattern in the transition region is then determined based at least in part on the artifact component. Once the ring-down pattern is identified, at least some of (and preferably substantially all of) the artifact component is filtered from the collected return signal based on the ring-down pattern.
The transition region is preferably identified by examining amplitude patterns in the collected return signal. For example, the signal may be analyzed to determine a rapid change from high amplitude to low amplitude. In many cases, the return signal will include a low frequency, high amplitude pattern which is indicative of the ring-down artifact, and a high frequency, low amplitude pattern which is indicative of blood. The point at which such a change is detected is referred to as a transition point and divides the signal into the transition region and a target or blood region.
Optionally, spectral patterns in the collected return signal may also be examined. Use of the spectral patterns can assist in identifying the transition region after the transition point has been identified or approximated.
Conveniently, a catheter is introduced into a body lumen and an ultrasonic source is excited within the catheter to emit the ultrasonic signal. In another aspect, the artifact component is enhanced so that the artifact component is readily identified. This may be done mechanically by repositioning the ultrasonic source. Enhancement may also occur electronically or by software. For example, the emitting and collecting steps may be repeated at different locations to obtain multiple scans. These scans are then convolved to dynamically enhance a pattern of ring-down artifacts as an accumulated ring-down pattern.
In another aspect, the ring-down pattern is stored for use in analyzing subsequent scans. The stored ring-down pattern for is then used for filtering where a ring-down-to-blood transition is not found in a subsequent scan. In still another aspect, the step of determining the ring-down pattern comprises obtaining a Fourier transform of the transition region and the blood region of the collected return signal and subtracting the transformed blood region from the transformed transition region.